Recently, packaging materials used in packaging have been required to have gas barrier properties enough to block, for example, oxygen, water vapor or other types of gases causing deterioration of a content from being passed through the packaging material so as to inhibit the deterioration of the content. Especially with foods, in order to inhibit the oxidation and deterioration such as of proteins, fats and oils and the like and keep their taste or freshness, it is necessary to prevent the influence of oxygen, water vapor or other types of gases acting to deteriorate the content on their passage through a packaging material of food. Thus, there has been required a packaging material having gas barrier properties sufficient to block these gases. With medical drugs needed to be handled under aseptic conditions, the influence of oxygen, water vapor and other types of gases acting to deteriorate the contents has to be prevented so as to suppress effective components from being deteriorated and keep their efficacies. Hence, packaging materials having gas barrier properties capable of blocking these gases have been required. Moreover, packaging materials having smell barrier properties have also been required for toiletry products, detergents and the like in association with diversified requirements for smell.
For barrier materials against oxygen and water vapor, there have been recently proposed a great number of barrier films wherein a nanoscale metal oxide film such as of silicon oxide or aluminum oxide is formed on a plastic substrate.
For the formation of such a metal oxide film on the surface of a plastic substrate (resin film), there are known, for example, methods making use of physical vapor deposition methods (PVD) including a vacuum film formation method, a sputtering method, an ion plating method and the like, or chemical vapor deposition methods (CVD) including a plasma chemical vapor deposition method, a thermochemical vapor deposition method, a photochemical vapor deposition method and the like.
Such barrier films as set out above are a kind of technology expected not only in the food package, but also widely in industrial applications from the standpoint that when comparing with existing barrier films using an aluminum foil, they have more improved or even excellent transparency and better barrier properties against oxygen and water vapor.
On the other hand, attention has been recently paid to the use of eco-friendly materials, which can be prepared in reduced amounts of fossil resources and can be treated at low energies when disposed along with a reduced discharge of carbon dioxide, in the context of the depletion of resources and the problems of warming induced by increased carbon dioxide in air, environmental pollution, and wastage. Under these circumstances, there has been expected the positive use of biomass resource-derived materials making use, partly or wholly as a starting material, of natural plants without use of starting fossil resources, or biodegradable materials that are decomposed into water and carbon dioxide in an environment. This is no exception in the packaging materials, and packaging substrates are actively being substituted from fossil resource-derived, synthetic materials to biomass resource-derived paper.
Further, there have been developed biomass-derived, gas barrier coating agents such as of polysaccharides including water-soluble starches and water-soluble cellulose derivatives. These are naturally derived and are thus improved or even excellent from the environmental and safety viewpoints. However, the coating materials of water-soluble polysaccharides are dependent on temperature and humidity, and deterioration of gas barrier properties under high humidity is unavoidable.
In order to solve these problems, a variety of attempts have been made. In Patent Literature 1, for example, it is described that gas barrier properties are developed by forming a cellulose nanofiber layer on a substrate and are improved by cross-linkage with inorganic compounds or by vacuum deposition of metal oxides. However, sheet materials having gas barrier properties have not been sufficiently investigated with respect to flexibility thereof, and it is thus assumed that flexibility is not adequate for practical applications associated with work and product design.